Atmospheric pollutants include those gases, particles, radicals and other molecules that make their way into the atmosphere from other sources or form in the atmosphere from the chemical reactions of other molecules and energy sources. In general, atmospheric pollutants can damage the atmosphere by contributing to the “greenhouse effect”, by breaking down the ozone layer, or by contributing to incidents of asthma and breathing problems. These pollutants are not merely confined to the outside, but can also be found in buildings. For example, many buildings have loading docks near an air intake system. When a truck pulls up to the loading dock, the truck exhaust can be pulled into the air intake system for a building and pollute the indoor air. There are also sources of atmospheric pollutants that originate from materials inside a building, such as carpet, paint, and commonly used chemicals.
Nitrogen oxides include a group of six compounds. Two members of this group, nitrogen oxide (NO) and nitrogen dioxide (NO2), often referred to as NOx, are reactive species that are considered problematic atmospheric pollutants and that are subject to regulatory control. The gases are regulated because of the large quantities produced through combustion and other chemical reactions and because of their adverse effects in atmospheric chemistry. More than 2 million tons of NOx were generated within the United States in 2011. Combustion typically produces 95% NO and 5% NO2. Nitric oxide, NO, is a significant reactive species in an atmospheric system, along with being present in several types of waste gases. It is the key component in the chain oxidation of organics, which is brought about initially by the radical product of the reaction of hydroxyl radical with organic compounds then adding an ozone molecule to the open radical site. NO scavenges an oxygen atom from the radical organic species to form NO2. In ambient air, there are other important mechanisms by which NO is quickly converted to NO2, including the following (wherein R is an organic moiety):2NO+O2→2NO2k298K=2.0×10−38 cm6molecule−2s−1 RO2+NO→RO+NO2k298K=7.6×10−12 cm6molecule−2s−1 HO2+NO→OH+NO2k298K=8.3×10−12 cm6molecule−2s−1 NO+NO3→NO2+O2k298K=1.8×10−14 cm6molecule−2s−1 NO+NO3→2NO2k298K=3.0×10 cm−11cm6molecule−2s−1 
A significant observation from the reactions above is that NO and ozone do not reside in the same system in significant concentrations, since NO reacts with ozone quite rapidly comparatively. NO can also react with RO and OH radicals, which have been called “nighttime storage reactions” for NO. Those two reactions are effective until dawn, because HONO and RONO will rapidly photolyze when the sun rises. Researchers have observed that when benzene and NO are in the same system, there is a direct reaction between the two where a series of nitrophenols are formed.
The product of most of the NO reactions, NO2, is also responsible for several important reactions in the atmosphere. The first significant reactions for NO2 are its reaction with ozone to form nitrate radical and oxygen or nitric oxide and two molecules of oxygen in reactions (14) and (15) below. This reaction is similar to the reaction of nitric oxide and ozone, in that neither molecule can simultaneously reside in the atmosphere in large concentrations. The nitrogen dioxide and ozone reaction has been attributed to the broad class of “nighttime chemistry” that NOx is responsible for in the atmosphere. NOx chemistry is important because many of its reactions do not require light, unlike several of the oxygen reactions. Ozone concentration in the atmosphere is at its lowest at nighttime, and therefore, NOx species can interact with other reactive species without automatically getting quenched by ozone. The significant reactions of NO2 in the atmosphere are as follows:

Reaction (13) is important environmentally because it is a source of nitric acid in the troposphere. Nitric acid contributes significantly to acid rain and fogs during the daytime, since most of the hydroxyl radical in the atmosphere is formed during daylight hours. Reactions (14) and (19) are chemically important because the highly reactive nitrate radical is formed.
Nitrate radical has been shown by researchers to be highly reactive, especially with organic compounds, such as simple alkenes and aldehydes. High nitrate radical concentrations have been spectroscopically observed in polluted urban areas. The primary time of day that the nitrate radical is most reactive is at night. The reaction of nitrate radical with the cresols and phenols is considered a significant sink for these organics at night. Nitric acid is formed in this reaction scheme, which is an undesirable product in the atmosphere.
Research is currently being conducted to review NOx production in air systems. One possibility for NOx compound formation is through the reaction of NNH and oxygen atoms. This scheme is as follows:NNHN2+H  (24)NNH+O2HNNOONNOOH→N2+HO2 or N2O+OH  (25)NNH+OHHONH→N2+H2O  (26)NNH+ON2+OH  (27)NNH+ON2O+H  (28)NNH+ONH+NO  (29)
Reaction (24) shows the initial formation of NNH and is rapid on both sides of the reaction leading to a quickly established equilibrium. Once NNH is formed in the system, reactions (25)-(29) proceed at relatively high rates of reaction. Therefore, there are new possibilities for gas-phase formation of NOx species in an air system.
Sulfur oxides (SOx) are compounds of sulfur and oxygen molecules. Sulfur dioxide (SO2) is the predominant form found in the lower atmosphere. It is a colorless gas that can be detected by taste and smell in the range of 1,000 to 3,000 micrograms per cubic meter (μg/m3). At concentrations of 10,000 μg/m3, it has a pungent, unpleasant odor. Sulfur dioxide dissolves readily in water present in the atmosphere to form sulfurous acid (H2SO3). About 30% of the sulfur dioxide in the atmosphere is converted to sulfate aerosol (acid aerosol), which is removed through wet or dry deposition processes. Sulfur trioxide (SO3), another oxide of sulfur, is either emitted directly into the atmosphere or produced from sulfur dioxide and is rapidly converted to sulfuric acid (H2SO4).
Most sulfur dioxide is produced by burning fuels containing sulfur or by roasting metal sulfide ores, although there are natural sources of sulfur dioxide (accounting for 35-−65% of total sulfur dioxide emissions) such as volcanoes. Thermal power plants burning high-sulfur coal or heating oil are generally the main sources of anthropogenic sulfur dioxide emissions worldwide, followed by industrial boilers and nonferrous metal smelters. Emissions from domestic coal burning and from vehicles can also contribute to high local ambient concentrations of sulfur dioxide.
Sulfur dioxide is a major air pollutant and has significant impacts upon human health. In addition the concentration of sulfur dioxide in the atmosphere can influence the habitat suitability for plant communities as well as animal life. Sulfur dioxide emissions are a precursor to acid rain and atmospheric particulates. Due largely to the US EPA's Acid Rain Program, the U.S. has witnessed a 33% decrease in emissions between 1983 and 2002. This improvement resulted in part from flue-gas desulfurization, a technology that enables SO2 to be chemically bound in power plants burning sulfur-containing coal or oil. In particular, calcium oxide (lime) reacts with sulfur dioxide to form calcium sulfite. Aerobic oxidation of the CaSO3 gives CaSO4, anhydrite. Most gypsum sold in Europe comes from flue-gas desulfurization. Sulfur can be removed from coal during the burning process by using limestone as a bed material in Fluidized bed combustion. Sulfur can also be removed from fuels prior to burning the fuel. This prevents the formation of SO2 because there is no sulfur in the fuel from which SO2 can be formed. The Claus process is used in refineries to produce sulfur as a byproduct. The Stretford process has also been used to remove sulfur from fuel. Redox processes using iron oxides can also be used, for example, Lo-Cat or Sulferox. Fuel additives, such as calcium additives and magnesium oxide, are being used in gasoline and diesel engines in order to lower the emission of sulfur dioxide gases into the atmosphere. As of 2006, China was the world's largest sulfur dioxide polluter, with 2005 emissions estimated to be 25.49 million tons. This amount represents a 27% increase since 2000, and is roughly comparable with U.S. emissions in 1980.
Sulfur dioxide is the product of the burning of sulfur or of burning materials that contain sulfur:S8+8O2→8SO2 
The combustion of hydrogen sulfide and organosulfur compounds proceeds similarly.2H2S+3O2→2H2O+2SO2 
The roasting of sulfide ores such as pyrite, sphalerite, and cinnabar (mercury sulfide) also releases SO2:4FeS2+11O2→2Fe2O3+8SO2 2ZnS+3O2→2ZnO+2SO2 HgS+O2→Hg+SO2 4FeS+7O2→2Fe2O3+4SO2 
A combination of these reactions is responsible for the largest source of sulfur dioxide, volcanic eruptions. These events can release millions of tons of SO2.
Sulfur dioxide is also a by-product in the manufacture of calcium silicate cement: CaSo4 is heated with coke and sand in this process:2CaSO4+2SiO2+C→2CaSiO3+2SO2+CO2 
The action of hot sulfuric acid on copper turnings produces sulfur dioxide.Cu+2H2SO4→CuSO4+SO2+2H2O
Sulfite results from the reaction of aqueous base and sulfur dioxide. The reverse reaction involves acidification of sodium metabisulfite:H2SO4+Na2S2O5→2SO2+Na2SO4+H2O
Treatment of basic solutions with sulfur dioxide affords sulfite salts:SO2+2NaOH→Na2SO3+H2O
Featuring sulfur in the +4 oxidation state, sulfur dioxide is a reducing agent. It is oxidized by halogens to give the sulfuryl halides, such as sulfuryl chloride:SO2+Cl2→SO2Cl2 
Sulfur dioxide is the oxidizing agent in the Claus process, which is conducted on a large scale in oil refineries. Here sulfur dioxide is reduced by hydrogen sulfide to give elemental sulfur:SO2+2H2S→3S+2H2O
The sequential oxidation of sulfur dioxide followed by its hydration is used in the production of sulfuric acid.2SO2+2H2O+O2→2H2SO4 
Carbon bed adsorption, or adsorption by another material, is a process that does not convert the components of waste gases to other compounds as part of the process. Adsorption is an effective way of reducing the concentration of components in a waste gas stream at a low flow rate.
The contaminated gas flows through the bed, where the components of the waste gas can be adsorbed onto the bed material. There are, however, several problems with carbon bed adsorption. First, the choice of the bed material is one of the critical factors in the success of the component removal. Activated carbon, molecular sieves, activated alumina, and activated silica are common bed materials, although activated carbon is commercially the material of choice. The composition of the bed material influences which waste gas component is adsorbed and which components pass through the system and into the outlet air stream. Therefore, it is helpful if the operator knows the contaminants of the air sample that is being cleaned.
Second, the adsorption technique does not break down the components of the waste gas into smaller and/or other compounds; it only collects them on the bed material. Once the bed becomes saturated, it is taken off line and cleaned. The cleaning process can involve simply steam cleaning the bed, or regeneration, or can involve using a solvent combined with steam cleaning to remove captured waste gas components. The waste products from this process are then collected and disposed of by an environmentally safe procedure. The most common procedure is to separate the waste gas components from the aqueous phase that was produced by the steam cleaning process. This is time consuming, labor intensive and costly.
Another problem with the adsorption technique is that it requires more than one bed in parallel and sometimes in series. The adsorption process requires beds in parallel so that when one bed becomes saturated, it can be taken off line and the other bed put into subsequent use. Sometimes, it becomes advantageous to put beds in series so that large concentrations of waste gas components can be removed. The operator can also put beds made of different material in series to target different combinations of waste gases. These adsorption beds are quite bulky, since their average depth is one to three feet, therefore this process can be undesirable if space is limited. The arrangement of beds in series and parallel add to the consumption of time, labor and money in cooling and cleaning of the waste and the bed material.
Absorption is the process by which part of a gas mixture is transferred to a liquid based on the preferential solubility of the gas in the liquid. This process is used most often to remove acid stack gases, but it is a complex and costly method of control and removal of other components of waste gases. The high cost of the process is based on the choice of the absorbent and the choice of the stripping agent. Absorption is limited in its utility and not widely implemented in small industrial settings.
Plasmas are electrical discharges that form between electrodes. There are five general classes of nonequilibrium plasmas that can be used in some capacity for chemical processing, including synthesis and decomposition: the glow discharge, the silent discharge, the RF discharge, the microwave discharge, and the corona discharge. Each class is specific based on the mechanism used for its generation, the range of pressure that is applicable during its use, and the electrode geometry.
While electrical discharges are effective in breaking down components of waste gases into other compounds and components, it is clear that in each of these discharge arrangements, they require a power source (in some cases a significant one), may not be able to handle industrial scale treatment without honeycombed and serial designs of the discharges, and are generally designed to combat complicated waste gas streams that comprise various components, including ozone, NOx and volatile organic compounds.
Wet scrubbing methods are conventionally employed for removal of particulates, SOx and NOx from waste gas streams. For waste gas streams that contain a significant amount of NOx, whether it was an original contaminant or the result of chemical conversion of a volatile organic component, conventional technologies, such as those described earlier, may not be able to efficiently handle the NOx load on an industrial scale. Conventional technologies for industrial scale NOx treatment typically treat the NOx with two or three stage wet scrubbing technologies. The most common currently used is a three stage process: Stage 1 converts NO into NO2. Stage 2 chemically transforms the NO2 into other nitrogen containing compounds. Stage 3 removes odors created in the second stage. Literature shows a number of chemical reactants, some of which were outlined earlier, that are utilized in this and other multi stage NOx treatment technologies. These include nitric acid and hydrogen peroxide, sodium hydrosulfide and hydrogen peroxide, or ozone gas and sodium chlorite solution, ferric salt solutions and others. All of these are relatively effective, but each has pronounced limitations in operating costs, equipment costs or removal efficiency.
Conventional research has described chlorine dioxide's ability to convert NO into NO2, which has typically been described in literature as occurring in a wet scrubbing apparatus according to equation 30 below. Researchers in this area also describe the use of sodium chlorite in water solution within a packed bed or tray type scrubbing or other wet scrubbing apparatus to convert NO2 into nitric and hydrochloric acid as described in equation 31 below.2NO+ClO2+H2O→NO2+HNO3+HCl  (30)4NO2+NaClO2+2H2O→4HNO3+NaCl  (31)
As shown in many of these conventional applications where waste gas volumes are small, NOx and SOx can be adsorbed on carbon and other porous solid materials or absorbed into liquids like sodium hydroxide and water. Although useful in small volume applications, the technologies are not economically practical for industrial applications that produce tens of thousands of cubic feet per minute of waste gas containing NOx and SOx. Catalysts provide another technical option; they can reduce NOx into nitrogen compounds that are not considered pollutants. Catalysts are effective on gas streams with small oxygen concentrations. Unfortunately most industrially produced NOx waste gas streams also contain high oxygen concentrations, so this technology is not applicable.
Methods known in the art for abating nitrogen oxides using, e.g., chlorine dioxide, include those of U.S. Pat. No. 4,119,702 to Azuhata et al., U.S. Pat. No. 3,957,949 to Senjo et al., and U.S. Pat. No. 3,023,076 to Ernst Karwat.